This invention relates to the upgrading of petroleum residua, and more particularly, to the production of marketable fuel.
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by a relatively high metals content. This occurs because substantially all of the metals present in the original crude remain in the residual fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper sometimes being present.
The high metals content of the residual fractions generally preclude their effective use as chargestocks for subsequent catalytic processing, such as catalytic cracking and hydrocracking, because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas, and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. F.-1000.degree. F. temperature and a pressure of 1-10 atmospheres. The economic value of the coke byproduct is determined by its quality, particularly its sulfur and metals content. Excessively high levels of these contaminants make the coke useful only as low-valued fuel.
Coking has long been the most important process for upgrading of resid. Because of worsening of crude quality and improvements in vacuum distillation and catalytic cracking technologies, the quality of coker feed has been deteriorating for years. At the present time, the low quality coke produced by some refineries has become difficult to market.
In U.S. Pat. No. 4,317,717, T. Y. Yan, "CO-PROCESSING OF RESIDUAL OIL AND COAL", residual oil is reacted with coal under visbreaking conditions to demetalize the residual oil. In "PRODUCTION OF LOW METAL AND LOW SULFUR COKE FROM HIGH METAL AND HIGH SULFUR RESIDS" Ser. No. 411,141, filed Aug. 25, 1982, T. Y. Yan, high quality coke is produced from demetalized residual oil which was visbroken with coal or other particulate solids. The visbroken stream is separated into overflow and underflow streams using a settler. The low metal overflow is charged into a coker to produce the desirable low metal coke. As described in that application, the underflow is recycled to the reactor where the particulate solids perform demetalation of the residual oil.
As a heavy fuel, the most difficult specification for the residua to meet is viscosity and/or pour point. For instance, the heavier marine diesel fuel requires a pour point of 30.degree. C. and viscosity at 100.degree. C. of about 60 CST, while the pour point of the resid from a typical coker feed is about 60.degree. C. In order to reduce the viscosity of residua to an acceptable level, light fractions, such as kerosene and light cycle oil are added as the cutter stocks. The quantities of cutter stocks required for typical residua are 35 to 50%. These cutter stocks are expensive relative to the residua. As a result, the visbreaking process has become more popular in recent years. The visbreaking process generates cutter stock from the resid itself by mild thermal cracking. However, the visbreaking often leads to fuel instability.
It is an object of the present invention to avoid the production of solid coke in residual oil upgrading.
It is another object of the present invention to produce a marketable fuel from heavy residual oil.
It is another object of the present invention to provide a residual oil upgrading process which is less expensive in both capital and operational costs.